This invention relates to imaging media. In a preferred form, it relates to supports for photographic, ink jet, thermal, and electrophotographic media.
In order for a print imaging support to be widely accepted by the consumer for imaging applications, it has to meet requirements for preferred basis weight, caliper, stiffness, smoothness, gloss, whiteness, and opacity. Supports with properties outside the typical range for xe2x80x98imaging mediaxe2x80x99 suffer low consumer acceptance.
In addition to these fundamental requirements, imaging supports are also subject to other specific requirements depending upon the mode of image formation onto the support. For example, in the formation of photographic paper, it is important that the photographic paper be resistant to penetration by liquid processing chemicals failing which there is present a stain on the print border accompanied by a severe loss in image quality. In the formation of xe2x80x98photo-qualityxe2x80x99 ink jet paper, it is important that the paper is readily wetted by ink and that it exhibits the ability to absorb high concentrations of ink and dry quickly. If the ink is not absorbed quickly, the elements block (stick) together when stacked against subsequent prints and exhibit smudging and uneven print density. For thermal media, it is important that the support contain an insulating layer in order to maximize the transfer of dye from the donor, which results in a higher color saturation.
It is important, therefore, for an imaging media to simultaneously satisfy several requirements. One commonly used technique in the art for simultaneously satisfying multiple requirements is through the use of composite structures comprising multiple layers wherein each of the layers, either individually or synergistically, serves distinct functions. For example, it is known that a conventional photographic paper comprises a cellulose paper base that has applied thereto a layer of polyolefin resin, typically polyethylene, on each side, which serves to provide waterproofing to the paper and also provides a smooth surface on which the photosensitive layers are formed. In another imaging material as in U.S. Pat. No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The biaxially oriented sheets described therein have a microvoided layer in combination with coextruded layers that contain white pigments such as TiO2 above and below the microvoided layer. The composite imaging support structure described has been found to be more durable, sharper, and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. In U.S. Pat. No. 5,851,651, porous coatings comprising inorganic pigments and anionic, organic binders are blade coated to cellulose paper to create xe2x80x98photo-qualityxe2x80x99 ink jet paper.
In all of the above imaging supports, multiple operations are required to manufacture and assemble all of the individual layers. For example, photographic paper typically requires a paper-making operation followed by a polyethylene extrusion coating operation, or as disclosed in U.S. Pat. No. 5,866,282, a paper-making operation is followed by a lamination operation for which the laminates are made in yet another extrusion casting operation. There is a need for imaging supports that can be manufactured in a single in-line manufacturing process while still meeting the stringent features and quality requirements of imaging bases.
It is also well known in the art that traditional imaging bases consist of raw paper base. For example, in typical photographic paper as currently made, approximately 75% of the weight of the photographic paper comprises the raw paper base. Although raw paper base is typically a high modulus, low cost material, there exist significant environmental issues with the paper manufacturing process. There is a need for alternate raw materials and manufacturing processes that are more environmentally friendly. Additionally to minimize environmental impact, it is important to reduce the raw paper base content, where possible, without sacrificing the imaging base features that are valued by the customer, i.e., strength, stiffness, and surface properties of the imaging support.
An important corollary of the above is the ability to recycle photographic paper. Current photographic papers cannot be recycled because they are composites of polyethylene and raw paper base and, as such, cannot be recycled using polymer recovery processes or paper recovery processes. A photographic paper that comprises significantly higher contents of polymer lends itself to recycling using polymer recovery processes.
Existing composite color paper structures are typically subject to curl through the manufacturing, finishing, and processing operations. This curl is primarily due to internal stresses that are built into the various layers of the composite structure during manufacturing and drying operations, as well as during storage operations (core-set curl). Additionally, since the different layers of the composite structure exhibit different susceptibility to humidity, the curl of the imaging base changes as a function of the humidity of its immediate environment. There is a need for an imaging support that minimizes curl sensitivity as a function of humidity, or ideally, does not exhibit curl sensitivity.
The stringent and varied requirements of imaging media, therefore, demand a constant evolution of material and processing technology. One such technology known in the art as xe2x80x98polymer foamsxe2x80x99 has previously found significant application in food and drink containers, packaging, furniture, and appliances. Polymer foams have also been referred to as cellular polymers, foamed plastic, or expanded plastic. Polymer foams are multiple phase systems comprising a solid polymer matrix that is continuous and a gas phase. For example, U.S. Pat. No. 4,832,775 discloses a composite foam/film structure which comprises a polystyrene foam substrate, oriented polypropylene film applied to at least one major surface of the polystyrene foam substrate, and an acrylic adhesive component securing the polypropylene film to said major surface of the polystyrene foam substrate. The foregoing composite foam/film structure can be shaped by conventional processes as thermoforming to provide numerous types of useful articles including cups, bowls, and plates, as well as cartons and containers that exhibit excellent levels of puncture, flex-crack, grease and abrasion resistance, moisture barrier properties, and resiliency.
Recently, a superior imaging support of high stiffness, excellent smoothness, high opacity, and excellent humidity curl resistance, comprising a closed cell foam core sheet and adhered thereto an upper and lower flange sheet has been disclosed in U.S. application Ser. No. 09/723,518, filed Nov. 28, 2001 by Dontula et al. Such an imaging support can be manufactured using a single in-line operation, and can be effectively recycled. However, such an imaging support can be subject to a high degree of static charge generation and accumulation during manufacturing, sensitizing, finishing and photofinishing, as compared to conventional resin-coated paper. The problem arises from the fact that unlike paper, which is inherently conductive because of its moisture and salt content, the foam based imaging support is hydrophobic and highly insulating, and, therefore, can readily become electrostatically charged. This static build-up happens because of friction with dielectric materials and triboelectrically chargeable transport means such as rollers during high speed conveyance of the support. An electrically charged support can result in static discharge through generation of sparks that poses fire hazards in the presence of flammable solvents at a typical coating site.
Conventional photographic resin-coated paper prints control static by the use of conductivity in the paper core in combination with an external antistat layer. This is achieved by the addition of salt and moisture internal within the paper base as well as a low conducting layer on the outer most backside layer. Such a means of controlling static is typically humidity dependent and can suffer from a number of problems in low humidity conditions. Such problems include static discharge, static marking of light sensitive layers, static cling that may result in print jams during conveyance as well as multiple sheet feed in other printing devices. Furthermore the addition of salt to the paper base of a resin-coated photographic print can also result in salts leeching into the processing chemistry that can cause problems by interfering with the processing of the chemical layers in a typical silver halide image layer. Furthermore the addition of salt may interfere with the ability of the paper base to resist penetration of the processing chemicals and may result in a stain on the edge of the print. With an all polymer imaging element there is no internal means of conveying or bleeding off charge and therefore a different means of controlling static and charge accumulation is necessary.
Furthermore the needs of an all synthetic print paper are different from that of a light sensitive film base negative working system and other paper based imaging systems. For instance the photographic speed for a silver halide print paper is several times lower than that of a film base system. The sensitivity of the film silver halide system is much higher than that of a slower print paper system. On the other hand the print paper products are typically manufactured at much higher speeds. This places additional and unique demands on the performance requirements for the antistat and charge control system as the photographic materials convey across rollers of varying composition at very high speed. As the web separates from the roller surface, residual charge accumulation builds up and may cause a static discharge as it reaches a threshold level. In traditional paper products, the conductivity is provided by a salt compound but as the paper is processed some of the salt is leeched from the external antistat and the conductivity is therefore reduced. Since the paper product has an internal antistat, any additional static or charge management needs are provided by the internal conductivity of the paper. In an all synthetic print paper, in which the antistatic properties are provided by an external-antistat it is important to provide static and charge management that does not substantially change after processing.
For non-light sensitive imaging elements the lack of an internal (within the core or base structure of the element) antistat or means to bleed off charge accumulation can result in an all synthetic print paper sticking to rollers and therefore causing jams and other conveyance problems as well as several sheets sticking together that can cause paper jams. In some imaging systems, the paper is heated and compressed and brought into contact with another web such as a dye donor sheet in thermal dye sublimation. This process can result in sheet to sheet separation sticking problems and therefore it is important to provide the proper static management of the webs and in particular the print web.
The management and control of charge is very complex and control of such forces is not only dependent on the imaging element manufacturing and processing systems requirements but the imaging element itself must be co-designed in order to optimize the overall performance of the system and the imaging element.
For imaging supports, particularly those containing photographic emulsion, sparking can cause additional problems, such as irregular fog patterns or static marks and degradation of image quality. The static problems have been aggravated by increase in the sensitivity of new emulsions, increase in coating machine speeds, and increase in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling.
A vast majority of antistats for photographic paper, e.g., those taught in U.S. Pat. Nos. 5,244,728, 5,683,862, 5,955,190, and 6,171,769, are usually not xe2x80x9cprocess-survivingxe2x80x9d, meaning that they lose their conductivity after wet chemical processing. This may be acceptable for normal photographic paper for any subsequent use, since the paper core provides a conductive means for charge dissipation. However, for imaging supports comprising a foam core, such antistats, which are not process-surviving, may lead to difficulties related to print sticking and dirt attraction, in a low humidity ambient.
Therefore, a careful control of the electrostatic characteristic of the imaging support is a crucial issue, particularly for those comprising a highly insulating foam core. In addition, the conductive means adopted for static control of these foam based imaging supports must satisfy all the requirement of conventional color paper products, including conveyance without dusting or track off, backmark retention, and spliceability.
There is a need for a composite material that can be manufactured in a single in-line operation and that meets all the requirements of an imaging base.
There is also a need for an imaging base that reduces the amount of raw paper base that is used.
There is also a need for an imaging base that can be effectively recycled.
There is also a need for an imaging base that resists the tendency to curl as a function of ambient humidity.
There is also a need for static control for successful manufacture, sensitizing, finishing, photofinishing and end use of such a base.
It is an object of the invention to provide a composite imaging material that overcomes the disadvantages of prior imaging base.
It is a further object of this invention to provide a composite imaging material that resists humidity curl.
It is another object to provide an imaging member that can be manufactured in-line in a single operation.
It is another further object to provide an imaging member that can be recycled.
It is an even further object to provide such an imaging member with an electrically conductive means to achieve superior electrostatic performance of the imaging base.
These and other objects of the invention are accomplished by an imaging member comprising at least one imaging layer, a base wherein said base comprises a closed cell foam core sheet and an upper and a lower flange sheet adhered thereto, wherein said imaging member has a stiffness of between 50 and 250 millinewtons, and is conductive. The invention also provides a method of forming a conducting imaging member comprising supplying a base wherein said conductive base comprises a closed cell foam core sheet having a thickness of between 25 and 175 xcexcm, adhering a flange material to each side of said foam core sheet, and adding at least one imaging layer, wherein said imaging member has a stiffness of between 50 and 250 millinewtons.
This invention provides a superior imaging support. Specifically, it provides an imaging support of high stiffness, excellent smoothness, high opacity, and excellent humidity curl resistance. It also provides an imaging support that can be manufactured using a single in-line operation. It also provides an imaging support that can be effectively recycled. Additionally, the imaging member is rendered electrically conductive by incorporating a conductive means. Moreover, such an imaging member fulfills other requirement for successful manufacture, sensitizing, finishing, photofinishing and end use.